Generally, most high sensitivity magnetic sensors used to measure magnetic field strength do not reach their full potential. The sensing technique utilized by high sensitivity magnetic sensors is typically hampered by noise constraints. A main component of this noise is “1/f noise,” also known as Flicker noise, which is a signal or process with a frequency spectrum that lessens at higher frequencies. Pink noise patterns are also referred to as “1/f noise,” and are found in semiconductors, music melodies, atomic clocks, and in nature, including the sounds of wind and waterfalls. 1/f noise occurs in almost all electronic devices, and results from a variety of effects. For applications where detection of low frequency phenomena is critical, 1/f noise is a major problem. Thus, there is a need to mitigate the effect of 1/f noise.
Interest is increasing in the development of miniature sensors for sensing magnetic fields, such as, for example, extraterrestrial, industrial, biomedical, oceanographic, and environmental applications. The trend in magnetic sensor design and development is toward smaller size, lower power consumption, and lower cost for similar or improved performance.
Currently, several types of magnetometers, which are magnetic sensors with external instrumentation, are in use. The least expensive and least sensitive devices have a resolution of approximately 10-1 Oersted (Oe)/Hz1/2 and typically are Hall-effect devices. These devices operate by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. However, such devices generally do not lend themselves for applications requiring greater sensitivity, such as required, for example, in brain scan devices and magnetic anomaly detection devices.
Flux gate magnetometers are more sensitive than Hall-effect devices; having a resolution of approximately 10-6 Oe/Hz1/2. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil, and are generally difficult to micro-fabricate. Additionally, flux gate magnetometers typically require a relatively large amount of power and accordingly do not lend themselves to a low-cost, compact, portable design. Another way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by factor of 1.5 to as much as 100. Flux concentrators are described further in U.S. application Ser. No. 12/536,213 filed Aug. 5, 2009, entitled “MEMS DEVICE WITH SUPPLEMENTAL FLUX CONCENTRATOR,” and U.S. application Ser. No. 12/541,805, filed Aug. 14, 2009, entitled “MEMS DEVICE WITH TANDEM FLUX CONCENTRATORS AND METHOD OF MODULATING FLUX,” hereby incorporated by reference.
A magnetic sensor (magnetometer) which addresses 1/f type noise is taught in Hoenig, U.S. Pat. No. 4,864,237, issued Sep. 5, 1989, the complete disclosure of which is herein incorporated by reference. Hoenig teaches an apparatus for measuring magnetic fields, which change only at extremely low frequency. Hoenig uses a SQUID (superconducting quantum interference detectors) magnetometer, which includes a superconducting flux transformer that inductively couples a detected signal into a DC SQUID sensor. The magnetometer of Hoenig may include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1 Khz. However, the limitations of this device, and others like it, include the need for a cryogenic operation, which inherently does not lend to relatively low cost, low power use. Although SQUIDS are more sensitive magnetometers having a resolution of approximately 10-10 Oe/Hz1/2, because SQUIDS include a superconducting element, these apparatus typically must include cooling means at liquid gas temperatures, making them extremely bulky and expensive to operate. Also, their relatively large size generally limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field. As such, it is common in magnetic sensors to place the sense element between two stationary flux concentrators to enhance the field. See, for example, U.S. application Ser. No. 12/536,213, filed Aug. 5, 2009, entitled “MEMS DEVICE WITH SUPPLEMENTAL FLUX CONCENTRATOR,” and U.S. application Ser. No. 12/541,805, filed Aug. 14, 2009, entitled “MEMS DEVICE WITH TANDEM FLUX CONCENTRATORS AND METHOD OF MODULATING FLUX,” hereby incorporated by reference.
Furthermore, magnetic sensors used to detect objects that move slowly typically possess considerably low frequency 1/f-type noise, which is an unwanted condition. Generally, there is a tendency for such devices, which have higher sensitivity, to also exhibit higher 1/f-type noise. This type of noise generally occurs when using magnetoresistive-type magnetic sensors.
Another problem arising with magnetic sensor usage occurs when detecting change in a signal due to the influence of a magnetic field. The signal change may be small relative to a background signal or signals; referred to herein as the “DC offset.” For example, in spin valve giant magnetoresistor sensors, the change is approximately 5-10%. For anisotropic magnetoresistance sensors the measurable change is even smaller. Extracting the measured signal from the DC offset requires using carefully constructed bridges and other techniques. A way of increasing the magnetic field is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50. See e.g., N. Smith et al., IEEE Trans. Magn. 33, p. 3358 (1997), the complete disclosure of which is herein incorporated by reference. An example of such a device is taught in Popovic et al., U.S. Pat. No. 5,942,895, issued Aug. 24, 1999, entitled “Magnetic field sensor and current and/or energy sensor,” the complete disclosure of which is herein incorporated by reference, which teaches the use of Hall effect sensors with flux concentrator components.
U.S. Pat. No. 7,185,541, issued Feb. 2, 2010, hereby incorporated by reference, entitled “MEMS Structure Support and Release Mechanism,” discloses a MEMS device and method comprising, inter alia, a MEMS structure adjacent to a SOI base; a sacrificial support operatively connecting the base to the MEMS structure; a suspension member operatively connecting the base to the MEMS structure. An embodiment in U.S. Pat. No. 7,655,996 further comprises a current pulse generator adapted to send a current pulse through the sacrificial support, wherein the current pulse causes the sacrificial support to detach from the MEMS structure.
In U.S. Pat. No. 7,046,002, a flux concentrator system is disclosed wherein the flux concentrators “focus” the magnetic field lines at the sensor location. The flux concentrators are free to move, driven by a comb drive, thus modulating the field at the position of the sensor. When the frequency of oscillation is in the kilohertz range, low frequency signals of interest that are normally obscured by the 1/f noise (which dominates at low frequencies) are effectively shifted to higher frequencies where 1/f noise is significantly lower. The sensor is stationary, the flaps oscillate, and comb drives and silicon springs are required.